This invention relates generally to a heat-wire type air flow measurement apparatus for measuring the air flow in the intake pipe of an engine, for example, and particularly to a heat-wire type air flow measurement device which is used to detect control data corresponding to the running state of an automobile engine in the electronic control of the engine.
When an engine is controlled electronically, a control device detects signals corresponding to the running state of the engine, and calculates the amount of fuel to be injected, ignition timing, etc. based on these signals. Examples of this kind of engine monitoring means are an engine speed detector, coolant temperature detector, exhaust gas temperature detector and throttle opening detector, etc. An example of means for direct detection of the operating state of the engine is intake air volume detecting means, of which the heat-wire type air flow measurement apparatus is well known.
With this kind of measurement apparatus a heat generating element having a thermal resistance characteristic is placed in the intake pipe and heating power is supplied to the element. The heat generating element is exposed to the cooling effect of the intake air flow so the thermal dissipation of the element is proportional to the amount of air flowing in the pipe. The velocity of the increase in temperature when the supply of heating power to the element is set is related to the volume of air flowing in the intake pipe, and, by monitoring this temperature increase, it is possible to measure the amount of air flowing in the intake pipe.
With this kind of measuring device, however, the element is placed in the intake pipe for long periods of time resulting in dust adhering to the device, which in turn causes variations in the thermal transmissivity of the element surface, erroneous measurement of air flow volume and makes it impossible to supply accurate data to the engine control system.
To solve this kind of a problem, U.S. Pat. No. 4,322,970, for example, discloses the burning off of this dust adhering to the element by raising its temperature to 800.degree. C. When the air flow measurement device is applied to an engine control system, the heat generating element is temporarily heated based on the instruction from the microcomputer constituting the control system. Normally this kind of burn-off command would be given when the engine is stopped, i.e., the ignition switch has been turned off.
Even after the operator of the vehicle has turned off the ignition switch, the power source for the control unit is temporarily left on to burn off the dust on the element, which requires that the power to the control unit be turned off after burn off is completed. Consequently, the control unit requires a complicated structure.